We have used dextran sodium sulfate (DSS) colitis model and the radiation injury model to explore the effects of PGE2 in the response to injury in intestinal epithelial cells. A combination of in vivo and in vitro studies revealed: 1.Radiation induces cell cycle arrest in epithelial cells and PGE2 induces a premature release from cell cycle arrest. Both AKT and MAP kinase pathways are involved in the effects of PGE2 on radiation induced cell cycle arrest. 2. Administration of DSS to COX-2-/- or Myd88-/- mice results in atrophy in the distal colon with complete loss of epithelial proliferation. This atrophy is not seen in WT mice given DSS. In COX-2-/- and Myd88 -/- mice the atrophy and loss of epithelial proliferation seen with DSS is rescued by PGE2. We have identified a population of colonic stromal cells which constitutively express COX-2 and secrete PGE2. In WT mice, but not in Myd88-/- mice, exposed to DSS these cells migrate to an area adjacent to the epithelial stem cells. These studies indicate that in response to DSS induced injury, a population of COX-2 expressing stromal cells migrate, in an Myd88-dependent manner, to an area adjacent to the stem cells and protect epithelial proliferation by secreting PGE2. 3. Based on our experience with the DSS colitis model and the migration of COX-2 expressing stromal cells as a mechanism of epithelial protection we have sought to determine if that process can be generalized to other forms of epithelial injury. Compared with WT mice Myd88-/- mice had a different pattern of radiation-induced apoptosis with an increase in the number of apoptotic cells and a distribution higher up the crypt. The increased apoptosis in Myd88-/- mice was reversed by PGE2. The central hypothesis of this proposal is that epithelial cell protection by endogenous PGE2 is an important component of the physiologic response to intestinal injury and is mediated by decreased apoptosis and escape from cell cycle arrest. To test this hypothesis we propose the following Specific Aims: 1) To define the mechanism by which PGE2 induces an early release from cell cycle arrest in intestinal epithelial cells subjected to radiation injury. 2) To define the mechanisms by which DSS induces cell cycle arrest and by which PGE2 rescues this arrest. 3) To define the mechanism for the increased radiation induced apoptosis in Myd88-/- mice and the rescue by PGE2.